1. Field of the Invention
The present invention relates to an analog-to-digital converter for use in cameras.
2. Description of the Prior Art
In camera circuits, various analog signals are generated and required to be converted into digital signals for digital indication of the exposure information or for digital exposure control. For example, an analog signal representative of the scene brightness is generated by a light measuring circuit. Such an analog signal, however, generally includes an undesirable factor caused by the difference in individual characteristics of the elements used in the circuit, such as the photosensitive element or logarithmic conversion element, the undesirable factor affecting the transmission of the true light measuring signal. Therefore, it is necessary to eliminate the undesirable factor from the analog signal generated from the light measuring circuit. For this purpose, a method is available in which an adjustable analog signal is generated and a subtraction between the analog signal of the light measuring circuit and the adjustable analog signal is carried out. Thus, a digital signal corresponding to the difference between the two analog signals is obtained with the undesirable factor eliminated.
As is exemplified above, in the camera circuit for digitally indicating an exposure information or for digitally controlling the exposure, a digital signal corresponding to the difference between two analog signals is frequently required. For this purpose, it is possible to provide an analog subtraction circuit for processing the two analog signals and a conventional A/D converter for converting the output analog signal from the analog subtraction circuit into the desired digital signal. Alternatively, the final digital signal may be obtainable by means of converting each of the two analog signals into corresponding two digital signals through a conventional A/D converter, and then processing the two digital signals through a digital subtraction circuit.
In any case, however, it has been necessary to use a subtraction circuit or the like in addition to the A/D converter.
The present invention can be distinguished from such prior art by considering FIG. 1 which represents an example of the prior art conventional A/D converter. In FIG. 1, transistors 102 and 104 and constant current source 106 constitute a constant voltage source to generate an output voltage corresponding to the base-to-emitter voltage V.sub.BE of transistor 102 at junction 110 between the base of transistor 102 and resistor 108. Between junction 110 and ground, there is connected a series circuit formed by resistor 108 and variable current source 112, which is controlled so as to generate a current corresponding to a digital output of counter 126. The change in current of variable current source 112 causes a change in the voltage level at junction 114 between resistor 108 and variable current source 112, junction 114 being connected to one terminal 118 of comparator 116. An analog signal to be converted into a digital signal is connected to the other terminal 120 of comparator 116. Comparator 116 inverts its output from "high" to "low" when the level of terminal 118 becomes lower than the level of terminal 120. Gate 124 transmits the pulses from clock pulse generator 122 to counter 126 until the output of comparator 116 is inverted from "high" to "low". Thus, the digital signal contained in counter 126 upon termination of pulse counting caused by gate 124 blocking the transmission of pulses corresponds to the analog signal at terminal 120, since the pulse counting is blocked when the level of junction 114 reaches the level of the analog signal in response to the current of variable current source 112 controlled by counter 126. After the A/D conversion is completed in the above manner, the result is digitally indicated by indicator 128.
In the prior art, U.S. Pat. No. 4,118,712 discloses an A/D converter designed to function in the manner described above.